Scelidosaurus: ready for its closeup at last
The
first complete dinosaur skeleton ever identified has finally been studied in
detail and found its place in the dinosaur family tree, completing a project
that began more than 150 years ago.
By
Sarah Collins
University
of Cambridge -- One hundred and sixty-two years ago, some fossilised bones were
collected from the shore beneath Black Ven at Charmouth in west Dorset. They
were sent to Richard Owen at the British Museum in London, who was at the time
the acknowledged expert on fossils in Britain – among many other achievements,
he had invented the word dinosaur.
These
bones clearly belonged to a dinosaur, but were a jumble of the remains of
several different animals. Owen encouraged the finder, James Harrison, to look
for more specimens in order to clarify matters.
Within
a year, Harrison had recovered a near-complete skeleton of one animal. Until
that moment, dinosaurs had only been known from teeth and a few scattered
bones, so their structure and appearance had been entirely speculative. Such
rarity had led to the extraordinary (and largely incorrect) concrete models of
dinosaurs built in 1853 and 1854 that can be seen today at Crystal Palace Park
in London.
The
world had its first dinosaur skeleton and it was in the hands of the man who
had invented the word.
So,
what did Owen do with this find? He published two short papers on its anatomy,
but many details were left unrecorded. He failed to reconstruct the animal as
it might have appeared in life, and made no attempt to understand its
relationship to other known dinosaurs of the time. In short, he ‘re-buried’ it
in the literature of the time, and so it has remained ever since: known, yet
obscure and misunderstood.
For
various reasons, the research staff of the Natural History Museum in London,
where the specimen is now stored, did not restudy Owen’s old dinosaur, even
though the skeleton had undergone decades of preparation to clean up its bones.
The
rocks in which this dinosaur’s bones were fossilised, known as ‘Blue Lias’ on
Dorset's Jurassic Coast, are around 193 million years old, close to the dawn of
the Age of Dinosaurs, making it a potentially vital specimen to understanding
how the major dinosaur groups evolved and how they relate to one another.
Over
the past three years, Dr David Norman from the University of Cambridge's
Department of Earth Sciences has devoted much of his time to preparing a
detailed description and biological analysis of this dinosaur, called Scelidosaurus, completing
a project more than 150 years in the making.
The
results of Norman’s work, published as four separate studies in the Zoological
Journal of the Linnean Society of London, not only reconstruct what Scelidosaurus looked
like in life, but reveal that it was an early ancestor of ankylosaurs, the
armour-plated ‘tanks’ of the Late Cretaceous Period.
In
the latter half of the 19th century Harry Seeley, who had been trained in
Cambridge by Adam Sedgwick, established a fundamental classification of
dinosaurs based primarily upon the shape of their hip bones: they were either
saurischians (‘lizard-hipped’) or ornithischians (‘bird-hipped’).
This
classification, first published in 1888, proved reliable: all dinosaur
discoveries seemed to slot neatly into one or other of these groupings. This
implied relationships between the major known dinosaur groups [Ornithischia,
Sauropoda and Thetropoda].
However,
in a 2017 paper, Norman and his former PhD students Matthew Baron and Paul
Barrett argued that these dinosaur family groupings need to be rearranged,
re-defined and re-named. In a study published in Nature, the researchers
suggested that bird-hipped dinosaurs and lizard-hipped dinosaurs such as Tyrannosaurus evolved
from a common ancestor, potentially overturning more than a century of theory
about the evolutionary history of dinosaurs.
Another
fact that emerged from their work on dinosaur relationships was that the
earliest known ornithischians first appear in the Early Jurassic Period. "Scelidosaurus is
just such an Early Jurassic dinosaur and therefore represents a
species that appeared at, or close to, the evolutionary ‘birth’ of the
Ornithischia," said Norman, who is a Fellow of Christ's College.
"Given that context, what was actually known of Scelidosaurus? The
answer: remarkably little!"
Norman's
study of all known material attributable to Scelidosaurus has
now been completed and reveals a host of firsts.
"Nobody
knew that the skull had horns on its back edge," he said. "It also
had several bones that have never before been recognized in any other
dinosaur."
"It
is also clear from the rough texturing of the skull bones that it was, in life,
covered by hardened horny scutes - a little bit like the scutes plastered over
the surface of the skulls of living turtles," said Norman.
Its
entire body was protected by skin that anchored an array of stud-like bony
spikes and plates.
Now
that its anatomy is well known, it has proved possible to examine the
phylogenetic position and potential relationships of Scelidosaurus.
It had been seen for many decades as an early member of the group that included
the stegosaurs (including Stegosaurus with its huge bony
plates along its spine and a spiky tail) and ankylosaurs (the armour-plated
‘tanks’ of the dinosaur era), but that was based on a poor understanding of the
anatomy of Scelidosaurus. Now it seems that Scelidosaurus is
an ancestor of the ankylosaurs alone.
So
why are Scelidosaurus remains found only at Charmouth?
"We don’t know," said Norman. "This dinosaur lived at a time
when most of the continents of the world were clumped together in a world we
now call Pangea. So, logically, animals in ‘Dorset’ would have been able to
roam the globe and their fossil remains should crop up elsewhere. But for the
moment we only know this dinosaur from this one location."
The
dinosaur bones found at Charmouth are located in the harder limestone bands
seen in the sea cliffs. The limestone bands are ‘diagenetic’ having been
created by calcium carbonate precipitation from groundwater flowing through
coarser sandy sediment in which these dinosaurs were buried. The coarser
sediments indicate brief periods of higher energy water flow – possibly created
by periodic flooding in the area. The floods washed some dinosaurs into the sea
where they drowned, became buried and eventually fossilised.
"It
is unfortunate that such an important dinosaur, discovered at such a critical
time in the early study of dinosaurs, was never properly described," said
Norman. "It has now - at last! - been described in detail and provides
many new and unexpected insights concerning the biology of early dinosaurs and
their underlying relationships. It seems a shame that the work was not done
earlier but, as they say, better late than never."
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